In With a Whimper: Depopulation and Longtermism, Geruso and Spears give the following argument for why most people who'll ever live may have already died:

• People are generally having children below replacement rate: 1.66 children per woman in the US, and total global annual births peaked in 2014.

• If you project this forward 300-600 years, annual births fall below ~10M.

• This would leave us with a global population around 560M.

• Only a minor disaster could be enough to wipe out humanity once our population is so low.

They include a pretty bold chart:

To be fair, pretty much any continuation of that chart into the future is wild, but the one they've ended up with seems especially so!

I don't find this argument very convincing for several reasons, but I want to focus on a specific one: even granting all their assumptions I think we'd see evolution for higher fertility long before we got down to 10M annual births.

The paper says:

But what, you might ask, about heritability (intergenerational transmission of high-fertility cultural practices)? Won't the Amish or some other high-fertility, perhaps religious, sub-population expand to be as many as we need? For several reasons, no. We have addressed this question at more length in Arenberg (2022). In the very long run (i.e., potentially after the coming few centuries of decline), two facts would have to be true for heritability to be a solution: First, fertility in a high-fertility sub-group would have to be high enough (certainly above two, for example). We've already seen above that the "high fertility" of high fertility subgroups has been declining over the decades. High fertility used to mean 6 children per woman. Now it means 2.5. Before long, it may mean 1.8. Second, the children of high-fertility parents would have to be very likely to remain in their high-fertility cultural group. Where researchers have studied the empirical magnitude of these intergenerational correlations as they have played out in actual practice, they have found them to be positive, but small—too small, in fact, for the high fertility group to make much of a dent in overall population. It turns out your kids might choose not to inherit your cultural practices and beliefs.

If cultural evolution isn't enough, what about genetic evolution? They cite their 2022 research note Intergenerational Transmission Is Not Sufficient for Positive Long-Term Population Growth which does consider both cultural and genetic fertility, but their model of the latter doesn't seem to me to address the main ways I'd expect genetic evolution to reverse global population decline. Let me give a sketch:

Humans vary a lot in the strength and timing of their desire to reproduce. Some people never have any interest in children, others feel deeply drawn to parenthood from a young age, and for others it varies. One common way that it varies is that many non-parents, as they get older, find themselves suddenly seriously drawn to having children. This suggests, though doesn't demonstrate on its own, that desire to have children can be and sometimes is strongly influenced by our biology.

If a strong biologically driven desire to reproduce were possible, though, why wouldn't we already have one? Historically, having such a desire probably didn't have much of an effect on how many kids you had. Given the lack of cheap reliable birth control and much lower levels of sexual education, evolution mostly gave us a strong desire to copulate. Combined with cultural taboos on finding alternative outlets for this desire, there wouldn't have been much selection on desire specifically to reproduce. With those conditions changed, however, I'd predict we're already seeing strong evolutionary selection for this desire among humans.

Why doesn't this show up in any of the data Geruso and Spears consider in their article? I think the problem is that they look at many groups and see declining fertility in all of them, but all of these groups are large enough that they include people with a wide range of genetically-driven desires for children. I'd predict that within each group that share could be increasing, but it's not visible in the overall numbers for any group because other factors are stronger for now. Maybe research on the heritability of family size (comparing twins, or checking whether some families are consistently larger or smaller than otherwise similar ones across generations) would help check this?

This is a pretty specific story, and I'm not an expert in the areas I'm talking about. If anyone could point me to more analysis on these questions I'd like to see it! For example, it could turn out that there isn't very much biological influence on family size, biological drives are mostly only able to push people from zero to one children but not higher, or a few dozen generations wouldn't be enough to have much of an effect.